At the end of their manufacture, semiconductor wafers are systematically inspected, at least in most industrial manufacturing processes. This is done, for example, to search for any defects, which generally occur in the form of heterogeneity of form at the wafer surface.
It is important that this inspection is, in addition to being reliable, fast, or at least that its duration remains compatible with the rest of the production process. In other words, since the inspection is integrated into the manufacturing process of the wafer, the inspection method that is used also affects the rate of production.
Some inspection methods are aimed, in particular, at obtaining measurements relating to the wafer.
On the whole, the measurement methods for semiconductor products are based on optical principles. The spatial wavelength of the measured characteristics form limits to what may be called a field of analysis. Standards applicable to semiconductor products, for example, those known under the name of SEMI standards (from the “Semiconductor Equipment and Materials International” association), define the terminology of the fields of analysis.
Wavelengths of between 15 mm and 300 mm thus reveal defects in the general form of the surface, referred to as flatness defects, whereas wavelengths of between 2 nm and 80 micrometers are used to characterize the surface roughness of the wafer.
The subject here is what is known in the technical field as nanotopography, that is, the assessment of variations in the wafer surface that exhibit wavelengths of between 200 micrometers and 20 nm. Such variations correspond to altitudes relative to a wafer reference plane of between a nanometer and a few tenths of a micrometer. In a way, nanotopography can be viewed as a field of analysis located, in terms of precision, between measurement of the surface finish and measurement of flatness.
What is sought, in particular, is an evaluation for each of multiple areas of the surface, what is known as the “peak-to-valley,” that is, the height that separates the highest point in the area under consideration from its lowest point.
In nanotopography, so-called “interferometric” techniques are conventionally used. The light reflected from a reference surface is made to interfere with the light reflected from a surface undergoing inspection. Recent developments involve a so-called “auto-interferometric” method, wherein the reference surface is made up of an area of the surface undergoing inspection, which is next to the area that is undergoing inspection. One then works from one area to the next, each time evaluating a surface area relative to an adjacent area.
On the whole, interferometric techniques give satisfactory results in terms of precision. Their weakness, however, lies in the time required for the acquisition of data and in their sensitivity to movement of the surface, in particular, to vibrations. In practice, the data acquisition operations limit the rate of inspection to around 40 to 60 wafers per hour. Given that today's production systems can manufacture about 100 wafers per hour, it can be seen why a nanotopography inspection station using interferometry cannot be usefully incorporated or joined onto a manufacturing station. Theoretically, it would be necessary to add at least two to three inspection stations to each manufacturing station in order to ensure that inspection operations do not reduce rates of production. Since inspection and manufacture must be carried out in clean rooms, prohibitive costs would result in terms of machine investment and of space that is occupied.
In practice, nanotopography is today only very rarely used systematically for each wafer produced.
The object of this disclosure is to improve that situation.